The preparation of dense alumina-based ceramic articles via a sol-gel process comprising an alpha alumina or alpha ferric oxide nucleating material is known in the art. For example, M. Kumagai and G. L. Messing spoke at the American Ceramic Society meeting of May 2, 1984 on this subject and later caused the publishing of a paper in November, 1984 in Communications of the American Ceramic Society entitled "Enhanced Densification of Boehmite Sol-Gels by Alpha Alumina Seeding". Other references on this subject include the following publications: "Microstructural Evolution in Sintering of ALOOH Gels", by Yarbrough et al., J. Mater. Res.,(4), 1987, pp. 494-515; "Enhancing Densification of 93% Al.sub.2 O.sub.3 - 7% MgO Triphasic Zerogels with Crystalline .alpha.-Al.sub.2 O.sub.3 and MgAl.sub.2 O.sub.4 Seeds," Komareni et al., J. Mater. Sci. Lett., 6, 1987, pp. 525-27; "Sapphire Whiskers From Boehmite Gel Seeded with from .alpha.-Alumina, "Jagota et al., 85, 1987, pp. 527-34; "Ceramics By The Solution-Sol-Gel Route", Roy, Science, 238, Dec. 18, 1987, pp. 1664-69; "Solid-Phase Epitaxy of Boehmite-Derived from Alumina on Hematite Seed Crystals," Messing et al., J. Am. Ceramic Soc., 72, (5), 1989, pp. 864-67; "Nucleation of Alpha Alumina in Boehmite Gel," Pach et al., J. Mater. Res., 5, (2), Feb. 2, 1990, pp. 278-85; U.S. Pat. Nos. 4,744,802 (Schwabel), 4,770,671 (Monroe), 4,623,364 (Cottringer et al.), 4,829,031 (Roy et al.), and 4,797,139 (Bauer), and European Patent Application Nos. 0293163 (Wood), published Dec. 7, 1988; and 0294208 (Wood et al.), published Nov. 30, 1988.
Although both Monroe and Cottringer et al. disclose the use of chromium oxide as a modifying additive in a sol-gel derived alpha alumina-based ceramic, they neither suggest nor teach its use as a nucleating agent.
Although Schwabel indicates that materials crystallographically similar to alpha alumina, in addition to alpha ferric oxide, are contemplated as alpha alumina nucleating agents, he neither specifies the use of alpha chromic oxide nor teaches the use of alpha chromic oxide as a nucleating agent. Further, he discloses the use of chromium oxide as a modifying additive, but does not suggest its use as a nucleating agent.
Several of the publications cited above suggest the use of chromium oxide (Cr.sub.2 O.sub.3) as a nucleating agent for the growth of alpha alumina from boehmite. None of these publications, however, disclose a method for the effective use of such a chromium-based seed or nucleating agent. For example, Bauer suggests the use of chromium oxide (Cr.sub.2 O.sub.3) as a nucleating agent for the growth of alpha alumina crystals from boehmite under appropriate conditions, he fails, however, to disclose or suggest such conditions. Roy et al. attempted to use Cr.sub.2 O.sub.3 as a nucleating agent and concluded that a Cr.sub.2 O.sub.3 seed has "hardly any" on the alpha alumina transformation temperature, because it appears that such a seed needs to exactly match the alpha alumina lattice.
Yarbrough et al. attempted to seed an alpha alumina precursor by introducing a high purity chromia powder into a xerogel. Diffuse reflectance infrared spectra of the calcined gel suggests that the oxidation state of all or all but a small amount of the chromium was +6, not +3. Further, the authors concluded that alpha Cr.sub.2 O.sub.3 does not increase nucleation frequency as much as might be expected. This article includes a photomicrograph of their best example which, reveals domains having an average diameter of about 3 micrometers. No density data were reported.
Wood et al. teach the use of an amorphous iron containing oxy-hydroxy polymer as a nucleating material for the alpha transformation in basic aluminum salt derived aluminas. Although the inventors disclose that nucleating materials prepared by the cohydrolysis of iron and chromium salts can be as effective at nucleating the alpha alumina transformation in basic aluminum salt derived alumina as the hydrous iron complex they do not teach or suggest the use of alpha chromic oxide (Cr.sub.2 O.sub.3) as an alpha phase nucleating agent.